The invention disclosed and claimed herein pertains to the field of viewing or imaging an object through a diffusive medium by providing a system which illuminates the object with projected picosecond coherent light pulses, and then receives light reflected from the object through an optical shutter which is opened for picosecond periods in synchronization with pulse projection. More particularly, the invention pertains to such systems which successively view or image light reflective surfaces included in different discrete viewing spaces, and then combine the images to provide a complete image of the object. The field of the invention includes the field of combining a gated pulse imaging system of the above type with an optical heterodyne imaging system to overcome the degrading effects of forward scatter and back scatter in viewing an object through a dynamic diffusive medium.
The range at which an optical system viewing through a diffusive medium may form an image of an object is substantially limited by the phenomenon of forward scatter, and if the object is illuminated by light projected from the viewing system, by the phenomenon of back scatter. A diffusive medium is a medium such as fog or sea water which contains refractive diffused particulate matter.
Forward scatter is the scattering of light transmitted from the viewed object to a viewing system as the result of random refractions by the particles, some of the scattered light being sensed by the viewing system. While some of the light transmitted from the object will reach the viewing system without interacting with any particles, and will therefore provide a clear image of the object, the sensed scattered light will degrade the image at the photodetector of the system. If the viewing system is more than a limited distance from the object, the scattered light will cause the image to be totally unviewable.
Forward scatter generally becomes a significant problem at distances which exceed 10 attenuation lengths, and makes an object completely unviewable at 15 attenuation lengths. For example, if sea water contains a large concentration of certain refractive micro-organisms, a conventional viewing device such as a television camera would be unable to distinguish an object located more than 15 attenuation lengths away. An attenuation length is the distance at which the unscattered power of transmitted light drops to e.sup.-1 of its initial value.
Back scatter is the scattering of light projected from a source of light by refractive particles of a diffusive medium, where some of the scattered light is directed back toward the source. It is clear that back scatter will further degrade an image of an object which is viewed by a system which must provide illumination for the object.
Conventional techniques for overcoming the detrimental effects of back scatter in a diffusive medium such as sea water include volume scanning and range gating. According to the range gating technique, successive pulses having time durations on the order of nanoseconds are employed to illuminate an object to be viewed, and a shutter which selectively allows light to be admitted to the system is operated in synchronization with the projection of the pulses so that the shutter is only open during periods on the order of nanoseconds. The shutter remains closed as a pulse travels to an object to be viewed so that backscatter generated thereby is not viewed by the system. However, as a pulse reflected from the object returns through the diffusive medium, it generates a substantial amount of forward scattered light. When the shutter is opened for a nanosecond period to receive the reflected pulse, the forward scattered light will also be received by the system.
The technique of range gating is exemplified in U.S. Pat. No. 3,467,773, 1969, to P. J. Heckman Jr., which discloses the use of an electronic shutter which may be opened for periods having an order of magnitude of 20 nanoseconds. However, in order to shutter out forward scattered as well as back scattered light generated by successive illuminating pulses, a shutter must be used which has a shutter speed in the range of picoseconds rather than nanoseconds. Present electronic technology is incapable of providing an electronic shutter which can operate with such speed.
A dynamic diffusive medium is a diffusive medium in which the particles are in continuous random motion. As far as is known, the only means for overcoming forward scatter in a non-dynamic diffusive medium is an invention by Applicant, disclosed in a previously filed patent application entitled "Coherence Length Gated Optical Imaging System" Ser. No. 937,655, filed Aug. 28, 1978. In addition, a system for overcoming forward scatter in a dynamic diffusive medium is disclosed in another application previously filed by Applicant entitled "Optical Heterodyne System For Imaging In a Dynamic Diffusive Medium" Ser. No. 930,283, filed Aug. 2, 1978. Such system discriminates between scattered light and unscattered light by recognizing that scattered light is shifted in frequency in a dynamic diffusive medium due to the Doppler effect. The system employs a narrow pass filter adjusted to the frequency of unscattered light to filter out frequency-shifted scattered light.